title: Ex vivo porcine model to measure pH dependence of chemical exchange saturation transfer effect of glycosaminoglycan in the intervertebral disc.
abstract: OBJECTIVE
  Studies have linked low pH and loss of glycosaminoglycan (GAG) in the intervertebral discs (IVDs) of patients with discogenic back pain. The purpose of this study is to determine whether the chemical exchange saturation transfer (CEST) effect of GAG (gagCEST) is pH dependent and whether it can be used to detect pH changes in IVD specimens. Iopromide, a Food and Drug Administration approved agent for CT/X-Ray, was also evaluated as a pH-sensitive CEST probe to explore the agents' potential to measure IVD pH.
  
  
  METHODS
  The pH dependency of the CEST effect of chondroitin sulfate (containing GAG) and Iopromide phantoms was investigated at 7 T. Z-spectra from porcine IVD specimens were acquired before and after manipulating the pH with sodium lactate. Iopromide was injected into the specimens and the calibration curve was used to determine the pH status.
  
  
  RESULTS
  Chondroitin sulfate showed a non-linear dependence of gagCEST effect with pH and gagCEST signal differences were detected in the specimens. The CEST effect of Iopromide resulted in a sigmoidal relation with pH and was used to measure pH.
  
  
  CONCLUSIONS
  gagCEST is sensitive to pH and enables investigation of the IVD pH status. Iopromide CEST is independent of the local GAG concentration and has the potential for measuring pH in the IVD.
authors.affiliation: Department of Radiology and Biomedical Imaging, University of California, San Francisco, San Francisco, California, USA.
authors.forename: Gerd
authors.initials: G
authors.lastname: Melkus
authors.forename: Michelle
authors.initials: M
authors.lastname: Grabau
authors.forename: Dimitrios C
authors.initials: DC
authors.lastname: Karampinos
authors.forename: Sharmila
authors.initials: S
authors.lastname: Majumdar
mesh_terms: D000818:Animals; D006025:Glycosaminoglycans / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007403:Intervertebral Disc / Q000737:chemistry*; D007472:Iohexol / Q000031:analogs & derivatives* / Q000737:chemistry; D008279:Magnetic Resonance Imaging / Q000379:methods*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013552:Swine; D014018:Tissue Distribution
chemical_list: D006025:Glycosaminoglycans; D007472:Iohexol; C038192:iopromide
keywords: CEST; Iopromide; chondroitin sulfate; gagCEST; intervertebral disc; pH